Sedimentation & Clarification
Learn how sedimentation and clarification remove floc and suspended solids from drinking water, including settling principles, basin operation, sludge removal, hydraulic loading, carryover, and troubleshooting.
Sedimentation and clarification remove suspended solids from water by allowing particles that are heavier than water to settle. In conventional drinking water treatment, these processes usually follow coagulation and flocculation and reduce the amount of material that reaches the filters.
Effective clarification is important because filters are designed to remove the particles that remain after pretreatment, not to receive the entire solids load from raw water. When clarification performs poorly, filter runs can become shorter, head loss can increase faster, and filtered-water turbidity can become more difficult to control.
Sedimentation and Clarification
Sedimentation is the physical settling of suspended particles under the influence of gravity.
Clarification is the broader treatment process used to separate suspended solids from water, commonly by sedimentation.
In many treatment plants, the terms sedimentation basin, settling basin, and clarifier are used for equipment that performs this separation.
Where Clarification Fits in the Treatment Train
A conventional surface-water treatment sequence commonly includes:
- coagulation;
- flocculation;
- sedimentation or clarification;
- filtration;
- disinfection.
Coagulation destabilizes fine particles. Flocculation combines them into larger floc. Clarification then provides conditions that allow the floc to separate from the water.
Why Particle Size Matters
Very small particles settle slowly. After successful coagulation and flocculation, particles become larger and usually settle more readily.
Poor floc formation can therefore produce poor clarification even when the clarifier itself is mechanically sound.
Settling Depends on More Than Particle Size
Settling behavior can be affected by:
- particle size;
- particle density;
- particle shape;
- water temperature;
- water viscosity;
- hydraulic conditions;
- floc strength.
Discrete and Flocculent Settling
Some particles settle without changing significantly in size. This is often described as discrete settling.
In drinking water clarification, floc particles may continue colliding and combining while settling. This is called flocculent settling.
As floc becomes larger, its settling characteristics can improve.
Clarifier Inlet Conditions
Water entering a clarifier should be distributed as evenly as practical.
Poor inlet hydraulics can create:
- short-circuiting;
- excessive turbulence;
- floc breakup;
- uneven solids loading.
Short-Circuiting
Short-circuiting occurs when part of the water moves through a basin much faster than intended.
Instead of using the available basin volume effectively, some water takes a shorter hydraulic path from inlet to outlet.
This can reduce effective settling time and increase solids carryover.
Detention Time
A simplified theoretical detention-time relationship is:
Detention Time = Basin Volume ÷ Flow
The units must be consistent.
Detention-Time Example
A clarifier has an operating volume of 500,000 gallons and flow through the basin is 1.0 MGD.
First express the flow in gallons per day:
1.0 MGD = 1,000,000 gal/day
Then:
Detention Time = 500,000 ÷ 1,000,000
Detention Time = 0.5 day
Convert to hours:
0.5 × 24 = 12 hours
The theoretical detention time is 12 hours.
Theoretical Versus Actual Detention Time
The theoretical calculation assumes the entire basin volume is used uniformly.
Actual hydraulic behavior can differ because of:
- short-circuiting;
- dead zones;
- uneven inlet distribution;
- sludge accumulation;
- poor outlet conditions.
Surface Overflow Rate
A useful clarification concept is the relationship between flow and the surface area available for settling.
A simplified expression is:
Surface Overflow Rate = Flow ÷ Surface Area
Depending on the units used, the result may be expressed as gallons per day per square foot.
Surface Overflow Example
A clarifier receives 1,500,000 gallons per day and has a surface area of 3,000 ft².
Surface Overflow Rate = 1,500,000 ÷ 3,000
Surface Overflow Rate = 500 gpd/ft²
This value describes hydraulic loading on the basin surface. Whether a particular value is appropriate depends on the clarifier design and operating requirements.
Higher Flow Changes Clarifier Loading
If clarifier surface area stays constant while flow increases, surface overflow rate increases.
Higher hydraulic loading can make solids separation more difficult, especially when:
- floc is weak;
- raw-water solids are high;
- inlet distribution is poor.
Flow Distribution Between Basins
Plants with multiple clarifiers should consider how total plant flow is divided among units.
An uneven flow split can overload one clarifier while another remains lightly loaded.
Outlet Weirs
Clarified water commonly leaves a sedimentation basin over outlet weirs or through collection structures.
Good outlet design and condition help distribute withdrawal across the basin.
Uneven Weir Flow
Uneven flow over outlet weirs can indicate:
- poor leveling;
- blocked sections;
- hydraulic imbalance;
- uneven basin flow distribution.
Weir Loading
A simplified relationship is:
Weir Loading = Flow ÷ Total Effective Weir Length
The purpose of the calculation is to relate basin flow to the available outlet length.
Observe the Clarifier Surface
Routine observation can provide useful process information.
Operators should look for:
- floating floc;
- solids carryover;
- uneven flow patterns;
- algae;
- surface debris;
- abnormal turbulence.
Clarified-Water Turbidity
Turbidity leaving the clarifier is an important process-control indicator.
A rising trend can warn of deteriorating treatment before the filters show a serious problem.
Clarified-Water Turbidity Is an Intermediate Indicator
Clarified-water turbidity does not replace filtered-water monitoring. It helps operators determine how effectively coagulation, flocculation, and settling are working before filtration.
Floc Carryover
Floc carryover occurs when floc leaves the clarifier with the clarified water rather than settling and remaining in the solids collection area.
Possible causes include:
- poor coagulation;
- poor flocculation;
- weak floc;
- excessive flow;
- hydraulic short-circuiting;
- sludge accumulation;
- abnormal basin hydraulics.
Poor Coagulation Can Look Like a Clarifier Problem
If particles were not properly destabilized upstream, the clarifier cannot make them settle simply by providing more basin volume.
Operators should troubleshoot the entire upstream process.
Poor Flocculation Can Also Cause Carryover
Small or weak floc may settle too slowly or break apart before reaching the sludge zone.
Review:
- flocculator mixing;
- flocculation time;
- coagulant dose;
- pH;
- raw-water quality.
Hydraulic Overloading
If flow through a clarifier becomes too high for existing conditions, settling performance may deteriorate.
High flow can:
- increase surface loading;
- increase turbulence;
- reduce effective settling opportunity;
- increase solids carryover.
Sludge Accumulation
Settled material collects in the lower portion of a clarifier and must be removed according to plant operating procedures.
Excessive sludge accumulation can:
- reduce useful basin volume;
- create solids carryover;
- cause odor;
- promote undesirable biological activity;
- interfere with sludge-removal equipment.
Sludge Removal
Clarifiers may use:
- mechanical scrapers;
- collectors;
- hoppers;
- pumps;
- valves and piping.
The exact equipment depends on basin design.
Do Not Let Sludge Removal Become an Afterthought
Good coagulation and flocculation can produce a large amount of solids. Those solids must be removed from the clarifier reliably.
Sludge Withdrawal Frequency
The appropriate withdrawal frequency depends on factors such as:
- raw-water solids;
- coagulant dose;
- plant flow;
- clarifier design;
- sludge characteristics.
Too Little Sludge Withdrawal
Possible effects include:
- excessive sludge blanket depth;
- solids carryover;
- septic or odorous conditions;
- reduced effective basin volume.
Excessive Sludge Withdrawal
Unnecessary withdrawal can waste water and increase the volume of residuals requiring handling.
Operators should base withdrawal on process conditions and plant procedures rather than assuming more withdrawal is always better.
Sludge Blanket
Some clarifier designs maintain or develop a measurable layer of settled solids.
Monitoring sludge depth can help determine whether solids are accumulating abnormally.
Sludge-Depth Measurements
Where applicable, sludge-depth measurements can help operators:
- adjust withdrawal frequency;
- identify scraper problems;
- detect uneven accumulation;
- prevent excessive solids buildup.
Floating Sludge or Floc
Material floating to the surface can indicate several different problems.
Possible causes include:
- gas formation in accumulated sludge;
- algae;
- hydraulic disturbance;
- light floc.
Operators should identify the cause before making treatment changes.
Temperature Effects
Water temperature affects viscosity and settling behavior.
Cold water can make treatment more challenging by affecting:
- coagulation reactions;
- floc formation;
- settling characteristics.
Density Currents
Differences in water temperature or solids concentration can create density-driven flow patterns within a basin.
These currents may affect hydraulic performance and effective settling.
Wind Effects
Outdoor clarifiers can be influenced by wind.
Strong wind can contribute to:
- surface currents;
- uneven flow distribution;
- movement of floating material.
Algae
Algae can affect clarifier performance, particularly in exposed basins with favorable light and nutrient conditions.
Possible operational effects include:
- surface accumulations;
- turbidity changes;
- taste and odor concerns;
- filter loading.
Tube and Plate Settlers
Some clarifiers contain inclined tubes or plates that increase effective settling area within a relatively small basin footprint.
These systems can improve particle separation when properly operated and maintained.
Tube or Plate Fouling
Accumulated material can interfere with performance.
Operators should inspect for:
- solids buildup;
- algae;
- uneven flow;
- blocked passages.
High-Rate Clarification
Some plants use clarification processes that differ from conventional rectangular or circular settling basins.
Regardless of equipment type, the basic operator objective remains the same: produce water with sufficiently low solids loading for the downstream treatment process.
Clarification and Filter Performance
Clarifier performance should be evaluated partly by what happens downstream.
Poor clarification can lead to:
- rapid filter head-loss development;
- short filter runs;
- more frequent backwashing;
- higher filtered-water turbidity risk.
Filter Problems May Begin Upstream
When filter performance deteriorates, operators should not automatically assume the filter itself is the cause.
Review:
- raw-water quality;
- coagulation;
- flocculation;
- clarified-water turbidity;
- clarifier hydraulics.
Storm Events
Storms can rapidly change source-water quality and clarifier loading.
Possible changes include:
- higher turbidity;
- higher suspended solids;
- different organic matter;
- higher plant flow.
Responding to a Raw-Water Turbidity Increase
Operators may need to review:
- jar-test results;
- coagulant dose;
- pH;
- floc formation;
- clarifier loading;
- sludge withdrawal;
- filter response.
Flow Changes
A clarifier that performs well at one flow may behave differently after a major flow increase.
Operators should consider hydraulic loading whenever plant production changes significantly.
Example: Clarified-Water Turbidity Rises Suddenly
Check:
- raw-water turbidity;
- chemical feed;
- pH;
- rapid mixing;
- flocculation;
- clarifier flow distribution;
- sludge-removal equipment.
Example: One Clarifier Performs Worse Than Another
If both units receive the same source water, compare:
- flow split;
- inlet condition;
- outlet weirs;
- sludge depth;
- mechanical equipment;
- visible hydraulic patterns.
Example: Filter Runs Become Short After Clarifier Problems
This is consistent with increased solids loading to the filters.
Correcting the upstream clarification problem can improve filter performance.
Example: Clarifier Has Good Floc but High Carryover
Review hydraulic and mechanical conditions such as:
- flow rate;
- short-circuiting;
- outlet distribution;
- sludge accumulation;
- basin equipment.
Example: Clarifier Has Very Small Floc
The primary problem may be upstream rather than in the sedimentation basin.
Review coagulation and flocculation before changing clarifier operation.
Example: Sludge Accumulates on One Side
Possible causes include:
- collector malfunction;
- uneven floor conditions;
- hydraulic imbalance;
- blocked sludge piping.
Example: Floating Material Appears After Long Sludge Storage
Excessive solids detention can allow undesirable conditions to develop in the sludge layer.
Review sludge-removal practices and equipment.
Mechanical Equipment
Clarifier performance depends on mechanical systems such as:
- drives;
- scrapers;
- chains;
- flights;
- sludge pumps;
- valves.
Mechanical Failure Can Become a Water-Quality Problem
A failed collector may allow solids to accumulate even though upstream chemical treatment is correct.
Routine Inspection
Operators should routinely observe:
- drive operation;
- unusual noise;
- sludge withdrawal;
- surface conditions;
- weir flow;
- clarified-water turbidity.
Process Records
Useful clarification records can include:
- plant flow;
- raw-water turbidity;
- coagulant dose;
- pH;
- clarified-water turbidity;
- sludge withdrawal;
- filter run time.
Use Trends to Find the Cause
For example, if clarified-water turbidity rises at the same time that raw-water turbidity increases, the plant may need a treatment adjustment.
If clarified-water turbidity rises while upstream conditions remain stable, investigate clarifier hydraulics or equipment.
Do Not Adjust Chemicals for a Mechanical Failure
Increasing coagulant dose will not correct:
- a failed sludge collector;
- a blocked outlet;
- severe short-circuiting;
- an overloaded basin caused by incorrect flow distribution.
Do Not Blame the Clarifier for Poor Coagulation
A sedimentation basin cannot reliably remove particles that were never properly destabilized and flocculated.
Common Sedimentation and Clarification Mistakes
- Assuming all clarification problems originate inside the clarifier.
- Ignoring coagulation and flocculation when floc carryover increases.
- Ignoring plant flow when basin performance changes.
- Allowing excessive sludge accumulation.
- Withdrawing sludge without considering actual process conditions.
- Ignoring uneven weir flow.
- Using theoretical detention time as if it represented exact hydraulic behavior.
- Ignoring short-circuiting and dead zones.
- Waiting for filter problems before responding to increasing clarified-water turbidity.
- Using chemical adjustments to compensate for mechanical equipment failure.
A Practical Clarifier Review
- Review raw-water conditions.
- Review coagulation and flocculation.
- Verify basin flow.
- Observe inlet hydraulics.
- Observe floc settling.
- Inspect the water surface.
- Check outlet-weir flow.
- Review clarified-water turbidity.
- Check sludge depth and withdrawal.
- Review downstream filter performance.
A Practical Solids-Carryover Review
- Verify the turbidity measurement.
- Check raw-water turbidity.
- Check coagulant feed and pH.
- Observe floc size and strength.
- Check clarifier hydraulic loading.
- Look for short-circuiting or turbulence.
- Check sludge accumulation.
- Inspect mechanical equipment.
- Review filter response.
A Practical Sludge-Removal Review
- Inspect sludge-removal equipment.
- Measure or evaluate accumulated sludge where applicable.
- Review withdrawal frequency.
- Review sludge characteristics.
- Confirm valves and piping are open and functional.
- Adjust operation based on actual solids accumulation.
What to Remember for the Exam
- Sedimentation uses gravity to separate settleable particles from water.
- Clarification commonly follows coagulation and flocculation and reduces solids loading to filters.
- Poor upstream coagulation or flocculation can cause poor clarifier performance.
- Clarifier inlet conditions should distribute flow while limiting excessive turbulence.
- Short-circuiting reduces effective settling opportunity.
- Theoretical detention time equals basin volume divided by flow.
- Actual basin hydraulics can differ from theoretical detention time because of short-circuiting and dead zones.
- Surface overflow rate equals flow divided by basin surface area.
- Increasing flow increases hydraulic loading when basin area remains constant.
- Clarified-water turbidity is an important process-control indicator.
- Floc carryover can result from weak floc, poor treatment, excessive flow, bad hydraulics, or sludge accumulation.
- Settled sludge must be removed at an appropriate rate.
- Excessive sludge accumulation can reduce effective basin volume and contribute to solids carryover.
- Uneven outlet-weir flow can indicate hydraulic imbalance or maintenance problems.
- Cold water can affect floc formation and settling performance.
- Tube and plate settlers increase effective settling area but require inspection and maintenance.
- Poor clarification can cause shorter filter runs and faster head-loss development.
- A chemical adjustment will not correct a mechanical clarifier failure.
- Troubleshooting should consider raw water, coagulation, flocculation, hydraulics, sludge removal, and downstream filtration together.
- Good clarification protects the filters by removing as much properly formed settleable floc as practical before filtration.